1. Field of the Invention
The invention relates to a method for producing high purity tertiary C.sub.4 and C.sub.5 olefins by the dissociation of the corresponding alkyl ethers and the subsequent dimerization of the olefins to produce high purity dimers thereof. Namely the present invention relates to a method for producing high purity isobutene and diisobutene by the dissociation of alkyl tertiary butyl ether and the dimerization of the isobutene from the dissociation and a similar dissociation of alkyl tertiary amyl ether and dimerization of the isoamylenes from the dissociation.
2. Related Art
Isobutene is a component of a C.sub.4 refinery stream. The separation of isobutene from the corresponding normal olefins by simple fractionation is extremely difficult, because of the closeness of their boiling points. In commercial processes as generally practiced, called the cold acid process, the isoolefin is selectively absorbed by sulfuric acid and the resulting isoolefin-containing sulfuric acid extract is then diluted and heated or treated with steam to separate the isoolefin. The diisobutene is a by-product of this separation.
The separation is achieved by virtue of the solubility of the isobutene in sulfuric acid. The remainder of the C.sub.4 stream is recovered for further treatment to separate the n-butenes and n-butanes. Although the sulfuric acid is highly selective for the isobutene, there will be some copolymer dimer from the iso- and n-butene formed.
The isoamylene is similarly difficult to separate from streams containing normal C.sub.5 olefins.
U.S. Pat. No. 4,242,530 discloses the removal of isobutene from a C.sub.4 stream and the production of diisobutene in a "catalytic distillation process", which uses a catalyst structure, in this case cation acid exchange resin in wire supported cloth bags, as a catalyst and distillation component to carry out the reaction and fractionation concurrently. This is an excellent means to remove isobutene and to produce a diisobutene equal to that produced by the cold acid process.
The "catalytic distillation process" has also been shown to be a useful method for reacting an alcohol and isobutene to produce ether, e.g., methanol to produce methyl tertiary butyl ether (MTBE), U.S. Pat. Nos. 4,232,177 and 4,307,254. The etherification is known to be a reversible reaction and is forced to completion by the removal of product concurrently with the reaction. Similarly, U.S. Pat. No. 4,232,177 shows the use of the catalytic distillation process to dissociate ether, i.e., MTBE to produce methanol and isobutene and diisobutene. The catalytic fractionation is a combination liquid phase/vapor phase process just as any distillation is. The product isobutene, however, is contaminated with methanol since isobutene forms an azeotrope with methanol.
Commonly assigned U.S. patent application, Ser. No. 234,653 filed Feb. 17, 1981, discloses the preparation of the ethers of isobutene and isoamylene in the catalytic distillation process using C.sub.1 to C.sub.6 alcohols.
It has now been found that the dissociation of alkyl tertiary butyl ether and alkyl tertiary amyl ether is best carried out in vapor phase, although in some cases the catalytic distillation environment is overall more efficient.
Both isobutene and diisobutene are of significant value having diverse applications; for example, isobutene is one of the comonomers for butyl rubber and diisobutene is an intermediate in the preparation of detergents. Isoamylenes have use as tackifiers in rubber manufacturer, in insecticides and demulsifiers for tertiary recovery of oil, and the diisoamylene is useful as gasoline blending stock.